The present invention relates to optical recording/reproducing apparatus and an optical head and an LD module used for the optical recording reproducing apparatus.
In the current situation where various types of optical discs are available on the market, optical recording/reproducing apparatus and an optical head that attain stable tracking of discs conforming to a plurality of specifications. Methods used in this case are roughly classified into: (1) a method where an RF signal is used to generate a tracking signal (hereinafter referred to as the TE signal), such as the phase difference detection method and the heterodyne method; (2) a method where a main beam is split into side beams for a TE signal on a disc, such as the 3-beam method and the differential push-pull method; and (3) a method that uses only a main beam without using an RF signal, such as the push-pull method. As shown in FIG. 4C, the push-pull method splits a detector 51 composed of a photo-detection device that receives a reflected light of a main beam spot into four devices by a split line in the track direction 51 and a split line in the radial direction. The push-pull method obtains a tracking signal TE=(A+D)xe2x88x92(B+C) via an arithmetic circuit 42 shown in FIG. 4D from output of each photo-detection device and uses the TE for tracking control, since TE=0 when a spot 50 is in the center of a track (pit) but TE greater than 0 or TE less than 0 when the spot is deflected rightward or leftward.
Of these approaches, the method under (1) cannot be applied to media that require tracking servo for an unrecorded area, such as the CD-R and DVD-R. The method under (2) has a disadvantage that a track pitch cannot simultaneously support a plurality of discs, because it is necessary to align a side beam split from a main beam to the tracking direction as accurate as in micrometers and the optimum value of the interval depends on the track pitch of the disc. On the other hand, the push-pull method under (3) has advantages: first, the method does not depend on the presence/absence of an RF signal; secondly, it does not require accurate angle adjustment and high accuracy of position to the center of rotation of a disc; and thirdly, there are no restrictions on the difference of the track pitch between discs. Thus the push-pull method has been in widespread use since the optical disc was commercially introduced.
Systems for obtaining a focusing error signal for an optical head according to the related art includes the knife edge system, the Foucault system, the beam size system, and the astigmatism system. For an optical head where a light source and a photo-detection device are mounted individually, the knife edge system and the astigmatism system are generally used. On the other hand, for an LD module where a light source and a photo-detection device are mounted in the same package, the hologram Foucault system and the beam size system are generally used.
In the push-pull method, position and intensity of the spot 50 on the detector 51 composed of photo-detection devices may vary in case an objective lens is driven by a tracking coil and is moved in radial direction of a disc relative to another optical system, or in case a disc has a tilt from the optical axis. This causes direct offset signal in the generated TE signal (Tracking Error). This signal variation is called DC offset.
When tracking servo is applied with a DC offset component contained, the tracking performance is considerably degraded, causing a possible out-of-tracking error. Thus, the push-pull method is typically used with means for removing the DC offset.
As means for removing the DC offset according to the related art, a method is known where DC offset to accompany the eccentricity of a disc is estimated in advance, and after appropriate leaning, the offset amount is corrected in performing tracking servo. Another method according to the related art is known where tracking performance in the direction of the thread of an optical head is upgraded to minimized the lens shift. Another method according to the related art is known where a mirror area is provided on a disc and tracking servo is applied while correcting the offset in the mirror section.
These methods require complicated signal processing, mechanism section with good response characteristics, or specially formatted discs. Thus, methods under (1) and (2) with simpler configuration and less subject to influence by offset are more often found in actual applications.
On the other hand, in a focusing error signal according to the related art overlap signals (generated when a beam spot crosses a track (calledtrack cross signals) with the decentering of an optical disc, which appears as a disturbance to obstruct focusing servo. While overlapping of the track cross signals is a major problem in the astigmatism system, this cannot be completely avoided in the other systems. Conventionally, in order to reduce overlapping of track cross signals, a special diffraction grating for shifting some of the phase of a side beam was used as in the Japanese Patent Publication No. 11-296875/(1999), or a detector was split into more devices and special arithmetic operation processing was used to remove disturbance in the focusing error signal, as in the Japanese Patent Publication No. 2000-82262.
The invention, in view of the problems of the related art, aims at providing an optical head, an LD module, and optical recording/reproducing apparatus equipped with tracking control means that does not require complicated signal processing or position adjustment and can easily remove offset via a simple configuration.
An optical head according to the first aspect of the invention is an optical head provided in optical recording/reproducing apparatus, said optical head splitting a single laser beam emitted from a light source into a plurality of side beams via a diffraction element and radiating the plurality of side beams on a disc thus using the side beams for tracking control, characterized in that the spot diameter of a side beam imaged on a disc is set to at least 2.5 times as large as that of the main beam.
In this way, setting the spot diameter of the side beam at least 2.5 times as large as that of the main beam allows the spot of the side beam to be radiated in a wide range over several tracks on the disc in the radial direction. Thus, the reflected light caused by the side beam contains a negligible track cross component caused by a beam spot crossing a track (component caused by the difference in intensity between a track groove and a land). In other words, since the spot diameter of the side beam is increased, the cut-off frequency of the optical transfer parameter (OTP) is shifted in the lower frequencies. This removes a higher track cross component in the spatial frequency (reciprocal of the track pitch) and obtains a signal containing a DC offset component alone caused by lens shift. According to the invention, design is made so that the spatial frequency component corresponding to the track pitch may be removed via filtering effects, by enlarging the spot diameter of a side beam at least 2.5 times as large as that of the main beam.
When a difference in output signals from photo-detection devices is obtained by causing the reflected light of the side beam to enter a detector composed of the photo-detection devices split along a split line in the tracking direction, the difference contains almost no signals caused by the track cross component. However, the objective lens has shifted in the radial direction relative to another optical system such as a light source or a detector. This generates a difference in intensity of the reflected light corresponding to the shift on the split photo-detection devices, which difference displays the amount of the DC offset.
On the other hand, the spot diameter of the main beam is univocally determined by the track (bit) width and the reflected light caused by the main beam naturally contains a track cross component as well as a DC offset. A track cross signal containing such a DC offset is detected by the detector of the reflected light of the main beam.
Thus, by substantially subtracting a DC offset signal obtained from the detector of the reflected light of the side beam from a signal containing DC offsets obtained from the detector of the reflected light of the main beam, a TE signal where DC offsets are removed is obtained.
In the detection of a focusing error signal, using a side beam having a large spot diameter to carry out the arithmetic operation according to the related art can conveniently remove a track cross signal alone via the aforementioned filter effects, while leaving a signal component called a sigmoid signal necessary for focus servo. Another advantage of this approach is that detection systems of a focusing error signal are not particularly restricted as long as the spot diameter of a side beam satisfies the aforementioned conditions.
The side beam needs not to be a true circle bit it may be an ellipse. In case the width of the disc in the radial direction is at least 2.5 times as large as the spot diameter of the main beam, the amplitude of the track cross component after the arithmetic operation can be maintained at least 90 percent that of the track cross signal to be obtained.
The upper limit of the spot diameter of the side beam is restricted because the side beam must be in size not overlapping with the main beam on a disc. The upper limit is further restricted by the photo detection area of the detector of the side beam. Of these restrictions, the latter is more severe. One side of the detector is typically set to about 150 micrometers or below. In a design where the 10-fold magnitude is obtained between a disc and a photo-detector device in an optical head, the spot diameter of the side beam is reasonably set to 15 micrometers or below. On the other hand, the spot diameter of the main beam on a disc is about one micrometer thus the spot diameter of the side beam is desirably 15 times or below the spot diameter of the main beam.
An LD module according to the second aspect of the invention is an LD module according to the first aspect of the invention, characterized in that a hologram device designed to give aberration to beams other than a zero-order light is used in a diffraction device splitting a beam as means for enlarging the spot diameter of a side beam imaged on a disc to exceed that of a main beam.
In this way, obtaining a side beam with the spot diameter enlarged via a diffraction device splitting a main beam requires the same number of parts and the same scale as apparatus using the differential push-pull method according to the related art, without increasing the number of parts.
Optical recording/reproducing apparatus according to the third aspect of the invention comprises an optical head according to the first aspect of the invention or an LD module according to the second aspect of the invention, characterized in that
detectors receiving disc-reflected lights of a main beam and side beams have split lines in the tracking direction of an image entering the detector and that
said optical recording/reproducing apparatus performs arithmetic operation processing on output signals from photo-detection devices split by said split lines of each detector concerning the main beam and the side beams to obtain a tracking signal.
Obtaining tracking via such a method allows application of an arithmetic circuit having the same configuration as that of the differential push-pull method according to the related art as an arithmetic operation method. This configuration can be implemented easily.
Optical recording/reproducing apparatus according to the fourth aspect of the invention is optical recording/reproducing apparatus according to the third aspect of the invention, characterized in that said optical recording/reproducing apparatus removes a DC offset signal component of a tracking signal by subtracting a signal obtained from a side beam via the push-pull method from a tracking signal obtained from a main beam via the push-pull method.
Configuration according to the fourth aspect of the invention can be easily implemented, the same as the third aspect of the invention.
Further, it is possible to easily obtain a signal containing a negligible track cross component by using a sub-beam having a large size to detect a focusing error signal.
Furthermore, it is possible to provide an LD module that can generate a focusing error signal containing a negligible track cross component by arranging a detector where the sub-beam returns and carrying out arithmetic operation on the signal.